The motion of many heavy duty vehicles such as trucks, buses and off-the-road equipment is stopped by an air brake system. The motion of many of these vehicles is also slowed down by a retarder used in conjunction with the air brakes. The useful life of the air brakes is increased by use of the retarder to slow down the vehicle because it decreases the amount of wear on the friction elements and other components of the air brakes.
Typically, the retarder is a torque converter connected by a suitable drive train to the drive wheels of the vehicle. Usually the vehicle is also driven by an engine through this same drive train. Sometimes the same torque converter is used both to couple the engine to the drive wheels for moving the vehicle and when the air brakes are applied to assist in retarding movement of and stopping the vehicle.
After the air brakes are initially applied and substantial braking of the vehicle has occurred, the retarder is also energized to assist in slowing down and stopping the vehicle. The retarder is turned fully on by an electro-mechanical control to assist in slowing down the vehicle. After the maximum braking effort produced by the air brakes begins to decrease, which usually occurs before the vehicle comes to a stop, the retarder is turned fully off.
This turning on and off of the retarder provides a sufficiently abrupt change in the rate at which the vehicle is slowed down that many drivers object to the feel of the brake and retarder system and many passengers in vehicles such as buses are uncomfortable.